Heat has been identified as a major cause for degradation of materials, such as insulating materials. For example, heat can cause erosion, i.e. loss of material. In high tension power systems, heat generated by dry band arcing is the principal cause of degradation of the insulation on the surface of insulators. (Secondary factors contributing to degradation include pollution, moisture, ultraviolet (UV) exposure and ozone.) Heavily degraded insulation in high tension power systems presents hazards, such as the possibility of a ground fault. Therefore, in developing and testing new materials such as insulating materials, it is often important to assess their relative heat resistance.
Conventional methods for ranking different insulating materials typically employ the Inclined-Plane Test (IPT) technique. For example, both the American Society for Testing and Materials (ASTM) and International Electrotechnical Commission (IEC) have established standard IPT methods, such as the ASTM D2303 method and the IEC 60587 method. In an IPT method, a flat test surface of the sample is oriented at an angle of 45° to a horizontal plane. A test solution containing contaminant is allowed to flow along a path down the test surface. A high voltage is applied across two electrodes disposed at two ends of the path, typically spaced apart by two inches. The electrical current between the electrodes generates heat, which causes degradation of the sample material at the surface by erosion and/or tracking. A track is a conductive path that may develop if the degradation residue contains a conductive material, typically free carbon. The length of a track on a sample as a function of time is monitored. In a first test, if a track has not developed, or has not developed to a pre-determined length, such as one inch, within a pre-determined time period, such as one hour, the applied voltage is increased. In a second test, the time to develop a track of specified length at a specified voltage is measured. The materials are then ranked according to their rates of track growth as measured by the noted tests.
The IPT technique has a few shortcomings. For example, it takes a long time to conduct an IPT test: a typical IPT test for one sample will last at least 10 hours. Indeed, to measure erosion, the ASTM D2303 standard recommends 24 to 48 hours of test time for each sample. It is difficult to provide the same amount energy to different samples in IPT tests, which reduces the reliability of the test. The results of IPT tests are also affected by many environmental and other factors, which may not be controllable and can also negatively affect the reproducibility of an IPT test. In addition, tests often have to be repeated due to the uncertainty in the test results, leading to increased testing time.
It has been suggested that an infrared laser can be used as an energy source to quickly and cheaply rank different materials, see A. S. Vaughan, “Polymer surfaces: designing materials to prevent or withstand discharge activity,” in Proceedings of Surface Phenomena Affecting Insulator performance, Ref. No. 1998/235, (1998), pp. 9/1-9/3 [“Vaughan”]. It has been suggested that the erosion damage caused by irradiation of an infrared laser could be quantified by direct measurement of the consequent pit depth. In one reported procedure, maximum pit depths were measured optically using a microscope, see I. L. Hosier et al., “Simulations of surface discharge damage in polymers using laser ablation and computational modelling techniques”, in Proceedings of International conference on Dielectrics and Insulation, G. Woynarovich ed., (1997), pp. 349-352 [“Hosier”]. However, the measured pit depths do not always accurately reflect the ranking of heat resistance of different materials. Hence, this method taught by Hosier is not reliable in many cases.
Therefore, there is a need for a fast, efficient, reliable, and inexpensive method or assembly for assessing relative degradation resistance of materials.